1. Field of the Invention
The present invention generally relates to apparatus and methods for completing a well. Particularly, the present invention relates to positioning a plug in a wellbore. More particularly, the present invention relates to a single-direction plug for use in well completions and drilling with casing applications.
2. Description of the Related Art
In the oil and gas producing industry, the process of cementing casing into the wellbore of an oil or gas well generally comprises several steps. For example, a string of casing is run in a wellbore to the required depth. Then, cement slurry is pumped into the casing to fill an annulus between the casing and the wellbore wall to a desired height. A displacement medium, such as a drilling or circulation fluid, is pumped behind the cement in order to urge the cement to exit the inside of the casing and enter the annulus. The cement slurry is typically separated from the circulation fluid by at least one cementing plug. Due to the difference in specific gravity between the circulating fluid and the cement slurry, the heavier cement slurry initially drops inside the casing without being pumped by hydrostatic pressure. After the height of cement slurry column outside the casing equals the height of the cement slurry column inside the casing, hydrostatic pressure must be exerted on the displacement fluid to force the rest of cement slurry out of the casing and into the annulus.
After the desired amount of cement slurry has been pumped into the annulus, it is desirable to prevent the backflow of cement slurry into the casing until the cement slurry sets and hardens. This backflow is created by the difference in specific gravity of the heavier cement and the generally lighter displacement fluid. One method for preventing the backflow of cement slurry into the casing involves holding constant the hydrostatic pressure on the displacement fluid in the casing until the cement slurry sets and hardens. This method, however, expands the casing and creates non-adherence of the casing to the hardened cement after the hydrostatic pressure in the casing is released and the casing string contracts. Another method of preventing the backflow of cement slurry involves placing a check valve in the lower end of the casing string to prevent the backflow of the cement slurry into the casing. The check valve may be run on a conventional casing string or pumped down the casing and latched into a float collar with a recess near the bottom of the casing string. Then, the cement slurry is pumped through the check valve. One problem with the use of a check valve in preventing the backflow of cement slurry is that flowing a cement slurry or other fluid through the check valve may damage the check valve and may prevent the check valve from functioning properly. In addition, installing a check valve, even in the open position, on a lower portion of a casing string can cause a pressure surge within the wellbore, thereby damaging surrounding hydrocarbon-bearing formations.
Recently, drilling with casing has become popular as a time saving way to complete a well. Drilling with casing involves using a casing string as a drill string to form a borehole and then using the same string to line the wellbore. Typically, a cutting member is placed at the lower end of the string and is later either retrieved or destroyed by subsequent drilling of another section of wellbore. One challenge of drilling with casing is providing a cementing apparatus in the string to facilitate the circulation of cement after the wellbore is formed. As described above, some type of one-way valve is typically used. However, because drilling fluid must be circulated through the string as the wellbore is formed, any valve in the string can hamper the circulation of fluid that is necessary for drilling
Therefore, a need exists for an improved cementing apparatus for use in completing wells. There is a further need for an improved method of positioning a plug in a wellbore. There is also a need for a downhole tool capable of positioning at a desired depth in the wellbore.